An exemplary conventional frequency output type hot-wire flow meter is described with reference to FIG. 12. The frequency output type hot-wire flow meter comprises a heating resistor 101 disposed in measurement target air flow, a fixed resistor 102 connected to the heating resistor 101 in series, a buffer circuit 103, and a voltage frequency conversion circuit 104. The target air mass flow is measured based on the heating resistor current I101 that flows through the heating resistor 101. The measurement principle of flow rate for the hot-wire flow meter is well known and the detailed description of the measurement principle is omitted herein.
The air flow rate voltage signal Vi proportional to the heating resistor current I101 is obtained from the heating resistor current I101. The air flow rate voltage signal Vi is supplied to the voltage frequency conversion circuit 104 through the buffer circuit 103. The voltage frequency conversion circuit 104 converts the air flow rate voltage signal Vi linearly to generate the output frequency Fo as described below.
An exemplary voltage frequency conversion circuit 104 is described with reference to FIG. 13. The voltage frequency conversion circuit 104 comprises two constant current sources 11 and 12, a switch 15, a switch change-over circuit 19 for generating a switch change-over signal, and an integrating capacitor 21 connected to the output side of the switch 15, and a buffer circuit 27.
Two constant current sources 11 and 12 receive the air flow rate voltage signal Vi and output the air flow rate current signal Ii that is proportional to the air flow rate voltage signal Vi respectively. Two constant current sources 11 and 12 may be, for example, a current mirror circuit. One of the two constant current sources is connected to the integrating capacitor 21 by means of the switch 15. When the first constant current source 11 is connected to the integrating capacitor 21, the air flow rate current signal Ii flows in such a direction that charges are accumulated in the integrating capacitor 21. As the result, the voltage Vic of the integrating capacitor 21 rises. When the second constant current source 12 is connected to the integrating capacitor 21, the air flow rate current signal flows in such a direction that charges are released from the integrating capacitor 21. As the result, the voltage Vic of the integrating capacitor 21 falls.
The changing rate of the voltage Vic of the integrating capacitor 21 is proportional to the air flow rate current signal Ii. In other words, the changing rate of the voltage Vic of the integrating capacitor 21 is proportional to the air flow rate voltage signal Vi.
Operation of the voltage frequency conversion circuit 104 is described with reference to FIG. 14. The switch change-over circuit 19 has an upper limit threshold voltage Vthh and a lower limit voltage Vthl(Vthh>Vthl) and compares the voltage Vic of the integrating capacitor 21 with the threshold voltage to generate a change-over signal. If the voltage Vic of the integrating capacitor 21 is lower than the upper threshold voltage Vthh, the switch change-over circuit 19 generates a change-over signal so that the first constant current source 11 is connected to the integrating capacitor 21. As the result, the voltage Vic of the integrating capacitor 21 rises.
When the voltage Vic of the integrating capacitor 21 reaches the upper threshold voltage Vthh, the switch change-over circuit 19 generates a change-over signal so that the second constant current source 12 is connected to the integrating capacitor 21. As the result, the voltage Vic of the integrating capacitor 21 falls. When the voltage Vic of the integrating capacitor 21 reaches the lower threshold voltage Vthl, the switch change-over circuit 19 generates a change-over signal so that the first constant current source 11 is connected to the integrating capacitor 21. As the result, the voltage Vic of the integrating capacitor 21 rises.
The buffer circuit 27 generates a voltage signal that corresponds to the voltage Vic of the integrating capacitor 21. Therefore triangular waves are generated as shown in FIG. 14.
As described hereinabove, the changing rate of the voltage Vic of the integrating capacitor 21 is proportional to the air flow rate current signal Ii. Therefore the gradient of the triangular wave is proportional to the air flow rate current signal Ii. The larger gradient of the triangular wave results in the larger frequency Fo of the triangular wave, and the smaller gradient of a triangular wave results in the smaller frequency Fo of the triangular wave. Therefore the frequency Fo of the triangular wave is proportional to the air flow rate current signal Ii. In other words, the frequency Fo of the triangular wave is proportional to the air flow rate voltage signal Vi. The frequency Fo of the triangular wave output from the voltage frequency conversion circuit 104 is represented according to the following equation.Fo=Vi/(R·C·dVth)=Ii/(C·dVth)   Equation 1
Wherein R denotes the resistance value of the fixed resistor 10 connected to the air flow rate voltage signal Vi, C denotes the electrostatic capacity of the integrating capacitor 21, and dvth denotes the rage of change of the voltage Vic of the integrating capacitor 21, namely the difference between the maximum value and the minimum value. R, C, and dvth are all constant values.
As represented according to the equation 1, the air flow rate voltage signal Vi is converted linearly to obtain the output frequency Fo in the voltage frequency conversion circuit 104 of the conventional frequency output type hot-wire flow meter. The period T of a triangular wave is represented according to the following equation.
                    T        =                              1            /            Fo                    =                                                    (                                  1                  /                  Vi                                )                            ·                              (                                  R                  ·                  C                  ·                  dVth                                )                                      ⁢                                                  ⁢                                                  =                                          (                                  1                  /                  Ii                                )                            ·                              (                                  C                  ·                  dVth                                )                                                                        Equation        ⁢                                  ⁢        2            
Curvature of an output of the conventional frequency output type hot-wire flow meter that is caused by the difference between individual hot-wire flow meters is described with reference to FIG. 15A, FIG. 15B, and FIG. 15C. FIG. 15A is a graph showing the relation between the actual air flow rate Vair and the air flow rate voltage signal Vi obtained from the heating resistor. The straight line M shows an exemplary standard characteristic of the frequency output type hot-wire flow meter. The curve X shows characteristic that deviates from the standard characteristic due to individual difference. In other words, the relation between the actual air flow rate Vair and the air flow rate voltage signal has a curvature deviated from the standard characteristic.
FIG. 15B shows the conversion relation between the air flow rate voltage signal Vi and the output frequency Fo. The straight line L shows the linear conversion relation between the air flow rate voltage signal Vi and the output frequency Fo, and shows the frequency conversion relation in the voltage frequency conversion circuit 104 of the conventional frequency output type hot-wire flow meter.
FIG. 15C shows the relation between the actual air flow rate Vair and the output frequency Fo obtained by converting the air flow rate voltage signal Vi according to the conversion relation shown in FIG. 15B. A curve M×L shown in FIG. 15C shows a result obtained by linearly converting the air flow rate voltage signal Vi obtained by means of the frequency output type hot-wire flow meter having the standard characteristic shown by the straight line M in FIG. 15A according to the straight line L shown in FIG. 15B. A curve X×L shown in FIG. 15C shows a result obtained by linearly converting the air flow rate voltage signal Vi by means of the frequency output type hot-wire flow meter having the curvature characteristic shown by the curve X in FIG. 15A according to the straight line L shown in FIG. 15B.
As it is obvious from the comparison between the curve M×L and the curve X×L, the frequency output type hot-wire flow meter having the standard characteristic shows a linear relation between the actual air flow rate Vair and the output frequency Fo, but on the other hand the frequency output type hot-wire flow meter having the curvature characteristic shows a non-linear relation between the actual air flow rate Vair and the output frequency Fo. For example, a product that shows characteristic of the straight line M×L may pass inspection, but a product that shows characteristic of the curve X×L may not pass inspection.
A standard characteristic is shown by the straight line M in FIG. 15A, but the standard characteristic may be a curve. The curve X shown in FIG. 15A is an exemplary curvature characteristic, and curvature characteristic can be various as shown by a curve Y.
The voltage output type air flow meter has been known as well as the frequency output type air flow meter. A technique for correcting the non-linear characteristic relation between the mass flow rate of the measurement target and the output voltage signal has been known. JP-A No. 190647/1999 discloses a technique of correction arithmetic using a microcomputer for the voltage output type hot-wire flow meter.
JP-A No. 337382/1999, JP-A No. 94406/1996, and JP-A No. 62012/1996 disclose techniques for correcting non-linear characteristic using a linearizing circuit for the voltage output type hot-wire flow meter.
However, these techniques are used to correct non-linear characteristic, but are not used to correct the curvature characteristic of output due to individual difference between flow meters. A negative feedback amplifier circuit using an operational amplifier corrects the output of the above-mentioned linearizing circuit. The curvature characteristic of output cannot be corrected by this technique because the flow rate signal and the output signal are in a linear relation.
[Patent document 1]
JP-A No. 190647/1999
[Patent document 2]
JP-A No. 337382/1999
[Patent document 3]
JP-A No. 94406/1996
[Patent document 4]
JP-A No. 62012/1996
As described hereinabove, the frequency output type hot-wire flow meter causes the curvature characteristic of output due to individual difference between fluid passages and heating resistors, and due to change of production line, production lot, and the likes.